Terephthalic acid is produced by a liquid-phase oxidation reaction of a p-phenylene compound such as p-alkyl benzenes, typically p-xylene. In general, the p-phenylene compound is subjected to a liquid-phase oxidation reaction in acetic acid as a solvent in the presence of a catalyst such as cobalt and manganese or in the presence of the catalyst and an accelerator such as a bromine compound and acetaldehyde to obtain a crude terephthalic acid, and then the resultant crude terephthalic acid is purified to obtain the aimed high-purity terephthalic acid.
However, since acetic acid is used as a solvent in the above reaction and the reaction product contains impurities such as 4-carboxybenzaldehyde (4CBA) and p-toluic acid (p-TOL), a high purification technique is required to obtain the high-purity terephthalic acid.
There are known various methods for purifying the crude terephthalic acid obtained by the above reaction, such as a method of dissolving the crude terephthalic acid in acetic acid, water or an acetic acid/water mixed solvent under high-temperature and high-pressure and then subjecting the resultant solution to a catalytic hydrogenation, a decarbonylation, an oxidation or a recrystallization, and a method of subjecting a dispersion partially dissolving terephthalic acid crystal to high-temperature immersion treatment.
In both the production of the crude terephthalic acid by the liquid-phase oxidation reaction and the purification thereof, the separation of the terephthalic acid crystal from the resultant slurry is finally needed. When the dispersion medium (first dispersion medium) of the reaction product solution obtained by the liquid-phase oxidation reaction is acetic acid and a different dispersion medium (second dispersion medium) such as water is used in the purification, it is required to first separate the crystal from the reaction product solution and then re-disperse the separated crystal in the second dispersion medium. When the first dispersion medium in the reaction product solution is the same kind as the second dispersion medium for the subsequent purification, a substantial part of the impurities such as oxidation intermediate, for example, 4CBA and p-TOL and coloring substances remain in the dispersion medium dissolved after a high-temperature purifying operation of the reaction product solution from the liquid-phase oxidation reaction or the starting slurry composed of the first dispersion medium and terephthalic acid crystal. If the reaction product solution from the liquid-phase oxidation reaction or the starting slurry composed of the first dispersion medium and terephthalic acid crystal is cooled to about 100° C. while allowing the impurities to be dissolved therein, the impurities are included into the terephthalic acid crystal, thereby failing to obtain the aimed high-purity terephthalic acid. Therefore, it is necessary to conduct the separation at high temperatures under high pressures to separate a high-purity terephthalic acid from the reaction product solution obtained by the liquid-phase oxidation reaction, the starting slurry composed of the first dispersion medium and terephthalic acid crystal or the slurry after the purification treatment.
A centrifugal separation has been most generally used for separating a slurry into a crystal and a dispersion medium, which is also extensively used in the separation of the reaction product solution obtained by the liquid-phase oxidation reaction or the starting slurry composed of the first dispersion medium and the terephthalic acid crystal.
In the centrifugal separation, the starting slurry composed of the first dispersion medium and the terephthalic acid crystal is introduced into a basket which is rotated at a high speed to allow the first dispersion medium to overflow from the upper portion of the basket and the crystal to move downwardly. It has been known that this method involves several problems caused by the limitation in the structures and functions due to the operation at high temperatures under high pressures.
Since the rinsing during the centrifugal separation and the rinsing of the separated crystal are difficult in this method, the amount of the first dispersion medium adhering to the crystal increases. Therefore, the centrifugally separated terephthalic acid crystal is made into a slurry by a further addition of a high-temperature fresh solvent, thereby needing an additional separation into the crystal and the dispersion medium. In addition, the high-speed rotation at high temperatures under high pressures necessitates a difficult and complicated maintenance of the centrifugal separator, to increase the production costs.
There has been recently proposed a method of separating the slurry into the first dispersion medium and the crystal and then re-slurrying the separated crystal in a different kind of a dispersion medium by using a single apparatus. In this method, a dispersion medium replacement apparatus utilizing a gravitational sedimentation of the terephthalic acid crystal is used. The starting slurry composed of the first dispersion medium and the terephthalic acid crystal is introduced into the apparatus from its upper portion, whereas the second dispersion medium is introduced from its lower portion. The replaced slurry composed of the terephthalic acid crystal and the second dispersion medium is mainly discharged from the lower portion of the apparatus, and the first dispersion medium is withdrawn mainly from the upper portion of the apparatus.
As the apparatus for the above method, there have been proposed various apparatuses such as an apparatus equipped with perforated plates (Patent Documents 1 and 2) and an apparatus having no packing structures therein (Patent Document 3).
However, these apparatuses commonly have a problem that the terephthalic acid crystal in the starting slurry is difficult to be uniformly dispersed in the horizontal direction when feeding the starting slurry composed of the first dispersion medium and the terephthalic acid crystal into the upper portion of the apparatuses. If the crystal fails to be uniformly dispersed in a horizontal direction, the mixing in the vertical direction of the replacement apparatus cannot be prevented. The performance of the dispersion medium replacement apparatus is governed by the prevention of the first dispersion medium fed to the upper portion from moving towards the lower portion. Therefore, the mixing in the vertical direction should be prevented.
If the terephthalic acid crystal is non-uniformly fed to the plate of the uppermost stage in the apparatus equipped with the perforated plates, a downward flow to a lower stage is induced particularly through the perforations where the terephthalic acid crystal is concentratedly fed. In contrast, an upward flow is induced through the perforations where the terephthalic acid crystal is not fed. Therefore, the mixing between the upper and lower sides of the plate is enhanced to reduce the plate efficiency. Further, the perforations of plates may be clogged by the non-uniform feed of the terephthalic acid crystal.
In addition, in the apparatus having no packing structure therein, the non-uniform feed of the starting slurry causes a severe mixing in the vertical direction. Therefore, a distributor for dispersing the terephthalic acid crystal more uniformly should be disposed in the apparatus. However, the known distributor having a number of pores suffers from clogging, thereby failing to stably operate the apparatus for a long period of time.    [Patent Document 1] GB 2014985    [Patent Document 2] JP 57-53431B    [Patent Document 3] JP 8-231465A